Island Arc

An island arc is a curved chain of volcanic islands that forms on the overriding plate above an oceanic subduction zone, parallel to and about 100 to 200 km behind the trench. The arc sits where the descending slab reaches roughly 100 km depth and dehydration triggers mantle melting; the curvature reflects the geometry of subduction on a sphere. Examples include the Aleutians, the Marianas, and the Japanese arc. Arc volcanoes erupt andesite, the rock that defines the Andesite Line bounding the andesitic Pacific rim from the basaltic interior. Arc-trench systems generate the largest subduction earthquakes.